SNR margin & router questions

I'm about 5km from the local exchange and use a Vigor 2600VGi router (price
about £150), with the ADSL service provided by Zen. I need the Vigor for
its VPN capability.

I see typically:-
448k Up Speed : 1440k Down Speed : 12.5 SNR Margin : 62.5 Loop Attenuation

I accept that the 62.5 dB attenuation is fair for the distance.

I tried a different router - an Edimax AR-7084A (price about £20) This
typically shows:-
448k Up Speed : 2464k Down Speed : 5.5 SNR Margin : 63.5 Loop Attenuation

Clearly the lower SNR margin allows a much greater download speed. So why
do the two routers behave so differently on the same line?

At ths same location I have another line with the ADSL service provided by
Demon. I see much the same performance difference between the two routers
on that line.

I've tried more modern versions of the Vigor router - for example a V2800
and V2820 - both these show much worse performance, typically:-

448k Up Speed : 800k Down Speed : 18.0 SNR Margin : 62.5 Loop Attenuation

Can anybody explain why a premium price router should behave so poorly
compared with an economy product?

In March this year BT broke some wires leading into the village and a good
many people were without any phone or ADSL service for several days. Before
that time, my Vigor router would report:-

448k Up Speed : 1900k Down Speed : 6.0 SNR Margin : 63.5 Loop Attenuation :

When BT fixed their wires, they did something which appeared to affect the
performance of the Vigor routers. Can somebody explain this for me?

Thanks.

--
Graham J

"Graham J" <(E-Mail Removed)> wrote:

>I see typically:-
>448k Up Speed : 1440k Down Speed : 12.5 SNR Margin : 62.5 Loop Attenuation
>
>I accept that the 62.5 dB attenuation is fair for the distance.
>
>I tried a different router - an Edimax AR-7084A (price about £20) This
>typically shows:-
>448k Up Speed : 2464k Down Speed : 5.5 SNR Margin : 63.5 Loop Attenuation
>
>Clearly the lower SNR margin allows a much greater download speed. So why
>do the two routers behave so differently on the same line?

You can't make such simple comparisons. The modem syncs at a speed which
gives an SNR margin set by the exchange. The default margin is 6db but the
exchange will increase it if the line looks unstable (frequent re-syncs)
which is often the case on longer lines.

The modem stays at that sync speed and as noise varies the SNR margin also
varies unless the margin gets so low that too many errors occur and the
modem will resync at a lower rate to restore the margin set by the
exchange.

You can only make comparisons by looking at sync rate and SNR margin
immediately after the modem has synced and with minimum delay between the
comparison in the hope that noise levels have not changed. Both modems
should sync with about the SNR margin set by the exchange and the
comparison will be in the sync rate achieved. From the SNR margins you
quoted it looks like you didn't do this.

That said the design and/or ADSL chipset used in the Drayteks isn't very
good on poor lines tending to sync at lower rates and resync more often
than some other cheaper modems.

If you really need the features in the Draytek I think they do a router
(2910?) that uses external modems, not sure what the requirements are for
suitable external modems.

It would be worth browsing the Draytek uk support forum
http://www.forum.draytek.co.uk/ where you will find some discussions about
modem performance and probably some talk about the router.
--

"nospam" <(E-Mail Removed)> wrote in message
news(E-Mail Removed)...
> "Graham J" <(E-Mail Removed)> wrote:
>
>>I see typically:-
>>448k Up Speed : 1440k Down Speed : 12.5 SNR Margin : 62.5 Loop Attenuation
>>
>>I accept that the 62.5 dB attenuation is fair for the distance.
>>
>>I tried a different router - an Edimax AR-7084A (price about £20) This
>>typically shows:-
>>448k Up Speed : 2464k Down Speed : 5.5 SNR Margin : 63.5 Loop Attenuation
>>
>>Clearly the lower SNR margin allows a much greater download speed. So why
>>do the two routers behave so differently on the same line?
>
> You can't make such simple comparisons. The modem syncs at a speed which
> gives an SNR margin set by the exchange. The default margin is 6db but
> the
> exchange will increase it if the line looks unstable (frequent re-syncs)
> which is often the case on longer lines.
>
> The modem stays at that sync speed and as noise varies the SNR margin also
> varies unless the margin gets so low that too many errors occur and the
> modem will resync at a lower rate to restore the margin set by the
> exchange.
>
> You can only make comparisons by looking at sync rate and SNR margin
> immediately after the modem has synced and with minimum delay between the
> comparison in the hope that noise levels have not changed. Both modems
> should sync with about the SNR margin set by the exchange and the
> comparison will be in the sync rate achieved. From the SNR margins you
> quoted it looks like you didn't do this.
>
> That said the design and/or ADSL chipset used in the Drayteks isn't very
> good on poor lines tending to sync at lower rates and resync more often
> than some other cheaper modems.
>
> If you really need the features in the Draytek I think they do a router
> (2910?) that uses external modems, not sure what the requirements are for
> suitable external modems.
>
> It would be worth browsing the Draytek uk support forum
> http://www.forum.draytek.co.uk/ where you will find some discussions about
> modem performance and probably some talk about the router.

I have a Linksys router that used to sync at 10M using Nildram LLU. I
changed ISP to O2 and the same router immediately sync'd at 8M with a 13dB
margin. I swapped to the supplied O2 box and it managed a connection of 13M
with a 6dB margin. Is there something that the O2 equipment does differently
with the O2 box than with other ADSL2+ routers? I can't explain why the
Linksys won't drop the margin to get a better rate, even when it has been
left on for a few days.

I also have a long noisy line and have tried several routers including a couple of draytechs.

By far the best results are with a BT2700 HGV (aka 2Wire). This out performs everything else I have
tried on my line. They are dead cheap on ebay.

See here fro much discussion on this router:
http://www.the-scream.co.uk/forums/t...highlight=2700

HTH - Adam





David Quinton wrote:
> On Tue, 21 Oct 2008 22:10:06 +0100, "Graham J"
> <(E-Mail Removed)> wrote:
>
>> I'm about 5km from the local exchange and use a Vigor 2600VGi router
>
> I use a 2600 and it improved noticeably when I flashed it to the Long
> Line UK firmware
>
> (but it was a couple of years ago so can't give details - someone will
> know...)

On Wed, 22 Oct 2008 10:32:27 +0100, Adam Lipscombe wrote:

> I also have a long noisy line and have tried several routers including a couple of draytechs.
>
> By far the best results are with a BT2700 HGV (aka 2Wire). This out performs everything else I have
> tried on my line. They are dead cheap on ebay.

Is there an equivalent ADSL modem that's wired? The 2700 looks good, but I
don't want wireless.
--
Peter.
You don't understand Newton's Third Law of Motion?
It's not rocket science, you know.

"David Quinton" <(E-Mail Removed)> wrote in
message news:(E-Mail Removed)...
> On Tue, 21 Oct 2008 22:10:06 +0100, "Graham J"
> <(E-Mail Removed)> wrote:
>
>>I'm about 5km from the local exchange and use a Vigor 2600VGi router
>
> I use a 2600 and it improved noticeably when I flashed it to the Long
> Line UK firmware
>
> (but it was a couple of years ago so can't give details - someone will
> know...)

Two years ago the SNR margin would often drop to 2dB (similar attenuation
at 62dB) and although the router would claim to have a connection it would
not carry any significant traffic - even pinging the Zen default gateway
would fail most of the time. Rebooting would always cure the condition, and
the SNR margin would stay at about 5dB for a few days. Speed was typically
1900kbits/sec

So I installed the Long Line firmware in the self-same V2600VGi router;
performance then became more reliable. The SNR margin varied between 4dB
and 8dB, attenuation was reported as 63.5 dB, and speed dropped slightly to
1800kbits/sec.

The only change since then is that BT broke the wires in March. And since
then the SNR margin has been 12dB to 14dB (loss 63 dB) and speed only about
1300kbits/sec.

I tried converting the router to standard firmware and now see no difference
whatever between the two versions of firmware.

--
Graham J

"Martin²" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) et...
> The Vigor 2600VGi is plain ADSL router, but should connect faster at BT
> std 6dB SNR. Trouble is BT has
> changed their DSLAM management to stick to higher SNR margin in order to
> make connections more stable. You may need to press your ISP to get BT to
> lower the margin.
> I presume the Edimax is ADSL2 or 2+, if so it can work with lower SNR and
> higher speed.
>
> You can set the SNR on Vigor 2800 (and I presume 2820), instructions can
> be found on Draytek.co.uk forum.
> Regards,
> Martin

Thanks. Looked through the forum back to mid 2005, seacing for "set snr
margin" :::

The topic at:
http://www.forum.draytek.co.uk/viewt...set+snr+margin

suggests that you can't set the SNR margin - it's in the wishlist ...

Similarly:

http://www.forum.draytek.co.uk/viewt...set+snr+margin

The term "codinggain" might be relevant ...

http://www.forum.draytek.co.uk/viewt...set+snr+margin

... says that it isn't supported in the chipset.

If you know of specific instructions can you please post a link?

--
Graham J

On Wed, 22 Oct 2008 14:33:11 +0100, Peter Crosland wrote:

>>> I also have a long noisy line and have tried several routers including a
>>> couple of draytechs.
>>>
>>> By far the best results are with a BT2700 HGV (aka 2Wire). This out
>>> performs everything else I have
>>> tried on my line. They are dead cheap on ebay.
>>
>> Is there an equivalent ADSL modem that's wired? The 2700 looks good, but I
>> don't want wireless.
>
> It is a simple change in the configuration to switch it off. There is no
> wireless version.
>
> Peter Crosland

Thanks Peter, that's useful to know.
--
Peter.
You don't understand Newton's Third Law of Motion?
It's not rocket science, you know.

"Martin²" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) et...
> To graham J:
> I said:
>>> You can set the SNR on Vigor 2800 (and I presume 2820),..
>
> but it can't be done on the 2600 range ;-( I have 2600VGi as well ).
> Regards,

I know it can't be done on the 2600 range.

But I can't find anything on the Vigor forum to show me how it can be done
on the 2800 or 2820. Please can you post a link to suitable instructions

--
Graham J

> > I'm about 5km from the local exchange and use a Vigor 2600VGi router
> > (price about £150), with the ADSL service provided by Zen. *I need the
> > Vigor for its VPN capability.
>

We are 9.6 km from the exchange and are running an ADSLnation X modem
straight out of the box on one line and it syncs ok. on my 2nd line i
have the same setup and it just wont sync..

speed isnt the best... 160 D 352 U 63.9db D 31.5db U snr
varies between 0.4 and 1 D and 7 U

but its better than dial up...

Ian H